WASHINGTON - There is no water left in the spent fuel pool of reactor 4 at Japan's Fukushima nuclear plant, resulting in "extremely high" radiation levels, the chair of the U.S. Nuclear Regulatory Commission warned Wednesday.

"In addition to the three reactors that were operating at the time of the incident, a fourth reactor is also right now under concern. This reactor was shut down at the time of the earthquake," said NRC chairman Gregory Jaczko.

"What we believe at this time is there has been a hydrogen explosion in this unit due to an uncovering of the fuel in the spent fuel pool," he said, noting the explosion happened several days ago but its effects were cause for concern.

"We believe that secondary containment has been destroyed and there is no water in the spent fuel pool and we believe that radiation levels are extremely high which could possibly impact the ability to take corrective measures."

On Wednesday, Japan ordered emergency workers to withdraw from its stricken nuclear complex amid a surge in radiation, temporarily suspending efforts to cool the overheating reactors. Hours later, officials said they were preparing to send the team back in.

Chief Cabinet Secretary Yukio Edano said the workers, who had been dousing the reactors with seawater in a frantic effort to stabilize their temperatures, had no choice but to pull back from the most dangerous areas.

``The workers cannot carry out even minimal work at the plant now,'' he said Wednesday morning, as smoke billowed above the crippled Fukushima Dai-ichi nuclear complex. ``Because of the radiation risk we are on standby.''

Later, an official with Tokyo Electric Power, which operates the plant, said the team had withdrawn about 500 yards (meters) from the complex, but were getting ready to go back in.

The nuclear crisis has triggered international alarm and partly overshadowed the human tragedy caused by Friday's 9.0-magnitude earthquake and the subsequent tsunami, a blast of black seawater that pulverized Japan's northeastern coastline. The quake was one of the strongest recorded in history.

Later Wednesday, national broadcaster NHK showed military helicopters lifting off to survey radiation levels above the complex, preparing to dump water onto the most troubled reactors in a desperate effort to cool them down.

But Edano has already warned that may not work.

``It's not so simple that everything will be resolved by pouring in water. We are trying to avoid creating other problems,'' he said.

``We are actually supplying water from the ground, but supplying water from above involves pumping lots of water and that involves risk. We also have to consider the safety of the helicopters above,'' he said.

Radiation levels had gone down by later Wednesday, but it was not immediately clear if the workers had been allowed back in, or how far away they had withdrawn. The workers at the forefront of the fight ― a core team of about 180 ― had been regularly rotated in and out of the danger zone to minimize their radiation exposure.

Meanwhile, officials in Ibaraki prefecture, just south of Fukushima, said radiation levels were about 300 times normal levels by late morning. While those levels are unhealthy for prolonged periods, they are far from fatal.

Days after Friday's twin disasters, millions of people were struggling along the coast with little food, water or heat, and already chilly temperatures turned to snow in many areas. Up to 450,000 people are staying in temporary shelters, often sleeping on the floor of school gymnasiums.

More than 11,000 people are officially listed as dead or missing, and most officials believe the final death toll will be well over 10,000 people.

In an extremely rare address to the nation, Emperor Akihito expressed his condolences and urged Japan not to give up.

``It is important that each of us shares the difficult days that lie ahead,'' said Akihito, 77, a figure deeply respected across the country. ``I pray that we will all take care of each other and overcome this tragedy.''

He also expressed his worries over the nuclear crisis, saying: ``With the help of those involved I hope things will not get worse.'' (AP, AFP)